Carl has asked a significant question that hasn't been dealt with yet. The current for an ion gun's actual ion beam current is usually measured with a faraday cup and bears little if any relationship to the ion gun's supply current, but is more a function of the gun design. In short, how well does your gun take the supply current and transform it to measurable ion current. A guess of 75% is just not right.

It also depends on where you intercept the beam for doing work in the vacuum environment as to how much of your actual "at extractor" beam current does what you want it to do on target.

Richard Hull

Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

The currents quoted above were currents feed to the anode not measured on the beam.
The 60-70% is just my best estimate based on papers written on anode layer sources, which seam to be pretty consistent in the percentage of total current that goes into ion beam current.

I don't have a Faraday cup yet, but when I get one build, I will post measured values.

I don't understand this concern with the longevity of the anode. Why would it be degraded at any sort of abnormal rate? Surely it only receives a relatively benign electron bombardment as they drift out of the ExB region (in circles [trochoids]), and do not even bombard it directly (i.e. not normal to the surface) but at a tangent as the exb orbits drift it across.

The farnsworth systems had to have extractors replaced almost weekly according to Gene Meeks and Steve Blasing. Their guns were fairly compact and "bright". They were not externally cooled either.

Errosion, spallation, deposition and melting are the culprits in gun degradation.

Richard Hull

Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

But this is, as literally described, a virtual anode? So one of its beneficial features is that the physical anode doesn't experience the same flow of particles as a pure electric discharge device would, as you describe. The magnetic field provides the necessary polarisation of charge that make this quite a different beast to ones that you describe, that need regular electrode replacement.

I'm not saying you are wrong, but I like to understand a basic mechanism than have hand-wavyness towards dissimilar devices as a means to support a view on how it works.

I don't see how any electrons will head straight towards the anode at full field potential, as they would in these other devices. Instead the electrons would creep their way towards the anode, gyroradius by gyroradius, then glance against it once close enough. Intuitively speaking, I get the feeling that this would be a whole load less harmful than in a discharge device.

Andrew
Nice work
your gut feeling for ion current could be right. It will be interesting to see a faraday cup downstream with a secondary electron suppression aperture in front biased -200 to -400 volts with respect to cup (to kill secondaries).

All our ion implanters use cold cathode penning ion sources and when tuned right, an arc current of 2mA in the ion source will give 1.85mA in the cup after the mass selection magnet. To add to Richards comments on wear we get around a 100 hours before the ion source has to be cleaned and ion exit aperture elements have to be replaced.

I was going to make the ion sources for my fusor similar to Carls RF driven one but this seems simpler and with the ions being produced in an annulus of significant diameter, eventual focus should result in more ions at the focus as until focus they are reasonably spread out so they shouldn't defocus due to similar charge effects.

As of now the casings for 2 injectors complete, and an additional 3 are almost complete. This will provide a total of 5 injectors, 4 for the fusor, and 1 for testing.

The injector base plate is a 2.75" CF flange with a 5kv rated MHV feed through for the HV and a 1/4" swagelok VCR fitting for gas feed through. The gas will be feed into the injector casing with a 1/8" OD ceramic tube(not shown). Both are TIG welded to the CF flange. The flange has a tapped 10-32 thread for the injector to screw into (with vented screws).

The base plate seems to work fine. The injector was tested for 30min at about 5ma discharge current and 800v. The opposing window across from the injector became slightly warm to the touch, but showed no damage. After the 30min run the injector body/magnet were measured to be at 53*C (132*F), well within the NdFeB magnet temperature limits.